The invention relates to a focus detector of the type utilizing the comparison of incident light, which is advantageous for use in a continuous photographing operation as may be achieved with a motor driven still camera or cine camera.
The present applicant has previously proposed a variety of automatic focussing apparatuses which can be applied to a continuous photographing operation as with a cine camera. The proposed apparatus allows an automatic focussing to be achieved continuously even if the distance to an object being photographed varies from time to time. One of such apparatuses is disclosed in Japanese Patent Application No. 81,558/1976, and includes a stationary and a movable reflecting mirror, each of which is adapted to receive incident light admitted through a pair of independent windows and passes it to a corresponding light receiving element for the purpose of comparison. Specifically, the focus detector disclosed in such prior application comprises drive means for performing a movement of a focussing lens independently from the rotation of the reflecting mirror, means for producing a first signal when the amounts of light fed by the reflection of the respective mirrors are equal to each other, means for producing a second signal when the distance to an object being photographed to which the foccusing lens is focussed becomes equal to the distance to the point of intersection between the optical axes of the optical systems comprising the pair of reflecting mirrors, and means for determining the direction in which the focussing lens is to be moved in accordance with the relative timing of the first and the second signal.
The described focus detector is capable of detecting the focus and hence achievinga focussing operation in following relationship with a moving object being photographed, and hence is usable in a continuous photographing operation as with a cine camera. However, the movable mirror undergoes a reciprocating movement, and since the relative timing of the first and the second signal is reversed during the forward and the reverse movement, it is necessary to perform the focus detection only during the forward or reverse movement. Because the speed of movement of the movable mirror in the foward direction is equal to that in the reverse direction, it follows that one-half the time required for the reciprocating movement of the mirror represents a waste time which has no contribution to the focus detection.